A Time Division Multiple Access (TDMA) system transmits and receives data in time slots. Well-known types of wireless TDMA systems are the European digital cellular telephone system known as the Global System for Mobile communications (GSM) and the U.S. digital cellular telephone system (e.g., IS-136). For GSM the use of TDMA implies that all of the transmit energy of the signal is concentrated in 1/8 of the frame time. The transmitted time slot is often called a TX burst.
Acoustic peripheral devices induce harmonics of these TX bursts. The result is an undesirable audible humming sound that emanates from the loudspeaker. The magnitude of the hum depends on a number of factors, such as the electrical construction of a hybrid in the peripheral device, the distance of the TDMA transmitter from the peripheral device, the mechanical construction of both devices, the strength of the received signal in the GSM transceiver that in turn determines the strength of the transmitted signal, the phase of the call (e.g., the call set-up phase and the call connected phase may have different TX levels), and certain GSM features (e.g., Discontinuous Transmission (DTX) and frequency hopping).
When the transmitter and the peripheral device can be mechanically constructed in a predetermined manner, the audible humming can be eliminated by preventing the induction of TX energy into the audio band circuits. However this is not always possible or desirable. For example, mobile station (e.g., cellular telephone) peripheral devices are typically sold as separate units that connect to a mobile station through a standard interface. As such, it is difficult or impossible to control the layout and relative positions of the transmitter and the audio band circuits.
The frequency of the induced TX signal corresponds to the frame time, wherein one time slot (and the TX burst) is transmitted per frame from the mobile station. For the GSM case one frame is 4.615 ms in duration, which equals a frequency of 216.667 Hz (i.e., about 217 Hz). The shape of the induced 217 Hz noise pulses is dependent on the mechanical construction, as well as the distance to the audio device, the orientation of the device, the construction of the coils in the speaker and microphone, etc., and can vary significantly with changes in these mechanical parameters. The nature of the undesirable disturbance in phase, shape and amplitude cannot thus be determined a priori.
Furthermore, because of the multi-frame structure in GSM and the multiplexing of different logical channels into a dedicated channel, not every frame will contain a TX burst. By example, Slow Associated Control Channels (SACCH) are inserted every 13th frame and IDLE frames are inserted every 26th frame (for a full rate traffic channel), resulting in drop outs of the disturbance every 120 ms. For half rate frames, every 26th frame contains the SACCH, but only every second frame contains a TX burst for traffic data. As such, further variability of the audio hum can occur.
Another factor that influences the cyclo-stationary disturbance arises from the Discontinuous Transmission (DTX) mode of operation. The DTX mode is used in order to reduce overall interference of multiple users in the system by transmitting speech data only when the user is talking. Furthermore, in GSM there exists a so-called hangover period of four speech frames, meaning that the DTX function does not mute the transmission immediately when no speech is detected (typically by a Voice Activity Detector (VAD)). In this case so-called Comfort Noise Update (CNU) frames are transmitted, as long as there is no speech, at the rate of 1 CNU frame every 480 ms. The DTX mode of operation is controlled by the base station in such a manner that it informs the mobile station to either use or not use DTX, or leaves the decision to the mobile station.
FIG. 1 illustrates one example of a disturbance 9 that is induced into the audio signal of a peripheral device 2 that is connected to a GSM Fixed Wireless terminal 1 via a 2-wire connection comprised of an output loudspeaker line 10 and input microphone line 11. The hybrid 3 of the peripheral device 2 changes the connection from 4-wire to 2-wire, and a hybrid 4 of the terminal 1 connects the signal back to a 4-wire line. The GSM disturbance is induced inside the peripheral device 2 and generates a disturbance in both loudspeaker signal 10a and microphone signal 11a. The interfered microphone signal 11a is converted into a digital signal by an A/D-converter 5. The digital signal is then coded by a Digital Signal Processor (DSP) 6 and fed to a Radio Frequency (RF) unit 7 of the terminal 1, which sends TX bursts 8 spaced 4.615 ms apart to the network. The DSP 6 is used to perform channel coding and modulation, as well as speech coding for the input microphone signal 11a. For the signal coming from the network opposite processing is done in order to generate an analog loudspeaker signal.
In the case illustrated in FIG. 1 the GSM disturbance 10a that is detectable in the output of the loudspeaker 10 (an audible hum of 217 Hz) cannot be measured directly. As a result, it is difficult to remove the unknown disturbance signal. Another disturbance occurs in the direction of the microphone line 11a. As a result the audible humming can also be detected in the earphone of the answering phone. However, this disturbance can be directly measured and suppressed.